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https://issues.apache.org/jira/browse/CASSANDRA-13120?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16017213#comment-16017213
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Benjamin Lerer edited comment on CASSANDRA-13120 at 5/19/17 10:35 AM:
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Right now, what {{CFHistograms}} expose is the number of SSTables on which we
do a partition lookup.
The partition look up can lead to skipping the SSTable (BF, min max, partition
index lookup or index entry not found) or not.
Nevertheless, partition lookups are not cheap. Especially when an index lookup
has to be done. Due to that, [~tjake] suggested to me, in an offline
discussion, to keep the
metric as it is and to add a new one {{mergedSSTable}} to track how many
SSTables have been actually merged.
The number of actually merged SSTables should also be the one used for the
{{Trace}} message and for determining if the {{SSTables}} must be compacted.
was (Author: blerer):
Right now, what {{CFHistograms}} expose is the number of SSTables on which we
do a partition lookup.
The partition look up can lead to skipping the SSTable (BF, min max, partition
index lookup or index entry not found) or not.
Nevertheless, partition lookups are not cheap. Especially when an index lookup
has to be done. Due to that, Jake suggested to me, in an offline discussion, to
keep the
metric as it is and to add a new one {{mergedSSTable}} to track how many
SSTables have been actually merged.
The number of actually merged SSTables should also be the one used for the
{{Trace}} message and for determining if the {{SSTables}} must be compacted.
> Trace and Histogram output misleading
> -------------------------------------
>
> Key: CASSANDRA-13120
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13120
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Adam Hattrell
> Assignee: Benjamin Lerer
> Priority: Minor
>
> If we look at the following output:
> {noformat}
> [centos@cassandra-c-3]$ nodetool getsstables -- keyspace table
> 60ea4399-6b9f-4419-9ccb-ff2e6742de10
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647146-big-Data.db
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647147-big-Data.db
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647145-big-Data.db
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647152-big-Data.db
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647157-big-Data.db
> /mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-648137-big-Data.db
> {noformat}
> We can see that this key value appears in just 6 sstables. However, when we
> run a select against the table and key we get:
> {noformat}
> Tracing session: a6c81330-d670-11e6-b00b-c1d403fd6e84
> activity
> | timestamp | source
> | source_elapsed
> -------------------------------------------------------------------------------------------------------------------+----------------------------+----------------+----------------
>
> Execute CQL3 query | 2017-01-09 13:36:40.419000 |
> 10.200.254.141 | 0
> Parsing SELECT * FROM keyspace.table WHERE id =
> 60ea4399-6b9f-4419-9ccb-ff2e6742de10; [SharedPool-Worker-2] |
> 2017-01-09 13:36:40.419000 | 10.200.254.141 | 104
>
> Preparing statement [SharedPool-Worker-2] | 2017-01-09 13:36:40.419000 |
> 10.200.254.141 | 220
> Executing single-partition query on
> table [SharedPool-Worker-1] | 2017-01-09 13:36:40.419000 |
> 10.200.254.141 | 450
> Acquiring
> sstable references [SharedPool-Worker-1] | 2017-01-09 13:36:40.419000 |
> 10.200.254.141 | 477
> Bloom filter allows skipping
> sstable 648146 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419000 |
> 10.200.254.141 | 496
> Bloom filter allows skipping
> sstable 648145 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 503
> Key cache hit for
> sstable 648140 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 513
> Bloom filter allows skipping
> sstable 648135 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 520
> Bloom filter allows skipping
> sstable 648130 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 526
> Bloom filter allows skipping
> sstable 648048 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 530
> Bloom filter allows skipping
> sstable 647749 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 535
> Bloom filter allows skipping
> sstable 647404 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 540
> Key cache hit for
> sstable 647145 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 548
> Key cache hit for
> sstable 647146 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 |
> 10.200.254.141 | 556
> Key cache hit for
> sstable 647147 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 564
> Bloom filter allows skipping
> sstable 647148 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 570
> Bloom filter allows skipping
> sstable 647149 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 575
> Bloom filter allows skipping
> sstable 647150 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 580
> Bloom filter allows skipping
> sstable 647151 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 585
> Key cache hit for
> sstable 647152 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 591
> Bloom filter allows skipping
> sstable 647153 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 597
> Bloom filter allows skipping
> sstable 647154 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 601
> Bloom filter allows skipping
> sstable 647155 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 606
> Bloom filter allows skipping
> sstable 647156 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 611
> Key cache hit for
> sstable 647157 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 |
> 10.200.254.141 | 617
> Bloom filter allows skipping
> sstable 647158 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 |
> 10.200.254.141 | 623
> Skipped 0/22 non-slice-intersecting sstables, included 0
> due to tombstones [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 |
> 10.200.254.141 | 644
> Merging data from memtables
> and 22 sstables [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 |
> 10.200.254.141 | 654
> Read 9 live and 0
> tombstone cells [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 |
> 10.200.254.141 | 732
>
> Request complete | 2017-01-09 13:36:40.419808 |
> 10.200.254.141 | 808
> {noformat}
> You'll note we claim not to have skipped any files due to bloom filters -
> even though we know the data is only in 6 files.
> CFHistograms also report that we're hitting every sstables:
> {noformat}
> Percentile SSTables Write Latency Read Latency Partition Size Cell Count
> (micros) (micros) (bytes)
> 50% 24.00 14.24 182.79 103 1
> 75% 24.00 17.08 315.85 149 2
> 95% 24.00 20.50 7007.51 372 7
> 98% 24.00 24.60 10090.81 642 12
> 99% 24.00 29.52 12108.97 770 14
> Min 21.00 3.31 29.52 43 0
> Max 29.00 1358.10 62479.63 1597 35
> {noformat}
> Code for the read is here:
> https://github.com/apache/cassandra/blob/cassandra-3.0/src/java/org/apache/cassandra/db/SinglePartitionReadCommand.java#L561
> We seem to iterate over all the sstables and increment the metric as part of
> that iteration.
> Either the reporting is incorrect - or we should maybe check the bloom
> filters first and then iterate the tombstones after?
> In this particular case we were using TWCS which makes the problem more
> apparent. TWCS guarantees that we'll keep more sstables in an un-merged
> state. With STCS we have to search them all, but most of them should be
> merged together if Compaction is keeping up. LCS the read path is restricted
> which will disguise the impact.
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